1. Field of the Invention
The present invention relates to an image forming apparatus for forming a color image for one frame by superposing a plurality of frame images.
2. Related Background Art
Conventionally, a color image forming apparatus for printing out color image data, e.g., a laser beam printer (LBP), forms a latent image in units of lines on a photosensitive body by a main scanning means (e.g., by irradiating a laser beam onto the photosensitive body by a rotary polygon), forms images in units of color components using developing agents of individual color components such as magenta (M), cyan (C), yellow (Y), black (BK), and the like, and transfers and superposes the images in units of color components onto a paper sheet fixed on a transfer drum, thus forming a color image.
Another image forming apparatus temporarily superposes images in units of color components formed on the photosensitive body onto an intermediate transfer body, and simultaneously transfers a color image on the intermediate transfer body onto a paper sheet.
In such conventional image forming apparatuses, the photosensitive body and transfer drum or intermediate transfer body are driven at given velocities in a direction (sub-scanning direction) perpendicular to the main scanning direction, and every time the transfer drum or intermediate transfer body completes one rotation, each color image is superposed on the paper sheet on the transfer drum or the intermediate transfer body. Also, in still another image forming apparatus, images in units of recording color components are formed on the photosensitive body to overlap each other, and the formed color image is simultaneously transferred onto a recording paper sheet.
In the conventional color image forming apparatus, the image quality of a color image often deteriorates due to offsets of the superposing positions of individual color images. In order to prevent the superposing positions of the individual color images from offsetting, a method of controlling the position upon superposing each color image is important.
For this purpose, for example, the following technique has been proposed. That is, a main scanning start signal (BD signal) is frequency-divided to obtain an integer number of BD signals during one rotation of the photosensitive body or intermediate transfer body, and these BD signals are used as reference clocks for a motor for driving the photosensitive body or intermediate transfer body. In this way, the laser scanning line is always scanned at an identical position independently of the number of rotations of the photosensitive body or intermediate transfer body, thus allowing easy color registration.
However, a BD signal is generated by scanning a laser beam onto the photosensitive body by a rotary polygon, and detecting it by a BD sensor located in the vicinity of the main scanning start position of one line of the photosensitive body or the corresponding position in the optical path. Hence, with the above-mentioned technique, since the photosensitive body or intermediate transfer body cannot be driven before the BD signal is detected, a time loss is produced from when a laser is turned on to detect a BD signal after the beginning of image formation until the BD signal is detected, thus prolonging the image formation time.
The laser beam, for BD signal detection, repetitively scans an identical position on the photosensitive body before the photosensitive body begins to rotate, Thus, photosensitive body unwantedly develops a so-called drum memory, and cannot be uniformly charged even via the charge removal and charging processes of the photosensitive body later, resulting in a short service life of the photosensitive body.
Furthermore, the laser must always be ON to detect the BD signal so as to drive the photosensitive body or intermediate transfer body, thus shortening the service life of the laser.